This invention relates to a photoreceptor for electrophotography for forming thereon an electrostatic latent image by a potential difference on its surface and more particularly to a method of stabilizing the surface potential of such a photoreceptor when it is uniformly charged at the beginning of an image forming process.
The surface of a photoreceptor for electrophotography such as those of a copier or a laser printer is uniformly charged to a potential of about 600-800 V before any electrostatic latent image is formed thereon. After an electrostatic latent image is formed by exposure of such a surface to an image forming beam of light, it is developed with toner to become a visible image which is thereafter transferred onto a copy paper sheet. In such an image forming process, uniformity in the photoreceptor potential significantly affects the quality of the image which is formed. If the photoreceptor surface potential is not uniform, or otherwise not appropriate, what are known as ghosts may appear in the formed image or the image density may turn out to be insufficient. In view of the above, it is currently a common practice to provide some optical fatigue to the photoreceptor by exposing its surface to a beam of charge-removing light prior to the process of uniform charging so as to stabilize the surface potential.
Optical fatigue gradually advances as a photoreceptor is used constantly in an image forming process but the photoreceptor gradually recovers from the fatigue by resting, that is, by remaining unused. According to conventional methods of stabilizing the surface potential by charge-removing light, however, a photoreceptor is always exposed to a same amount of charge-removing light. As explained above, a photoreceptor surface is in different conditions of optical fatigue, depending, for example, on whether it has been continuously in use or it has been unused for a long time. With such conventional methods by exposure to charge-removing light, therefore, photoreceptor surface potential cannot be kept at a uniform level.
Japanese Patent Publications Tokko 49-4337 and Tokkai 57-147782 disclosed methods of varying exposure to charge-removing light according to the period of rest but there was no proposal for control during a continuous operation or regarding periods of rest with power switched off. In other words, these publications did not teach how to control the exposure to charge-removing light although optical fatigue of a photoreceptor changes during a continuous use. Moreover, since prior art image forming apparatus are not provided with any timing means operating while power is switched off, they must either ignore rest periods while power is off or assume that the photoreceptor is completely recovered from fatigue whenever power is switched on after a period of rest, no matter how short. Those of the former type would have to somehow record the degree of fatigue immediately before power is switched off and there is a risk of erroneous correction after a long rest period. Those of the latter type, on the other hand, run the risk of insufficient correction when power has been off for only a short period.